Ethylene-bridged periodic mesoporous organosilicas with Fm3m symmetry

Abstract

Divalent surfactant [CH₃(CH₂)₁₅N(CH₃)₂(CH₂)₃N(CH₃)₃]²⁺2Br⁻ (C_16-3-1) was exploited as a structure directing agent (SDA) for the synthesis of highly ordered periodic mesoporous organosilicas (PMOs) from 1,2-bis(triethoxysilyl)ethane (BTEE) as organosilica source under basic conditions. The mesophase structure/symmetry and the physical parameters could be controlled/optimized by adjusting synthetic parameters such as surfactant and base concentration, hydrothermal aging temperature and time, and crucially the type (charge, shape, aliphatic tail chain length) of the surfactant. Cubic PMO[KIT-5]-n reveals a face-centred Fm3m symmetry similar to that of the previously reported purely siliceous material KIT-5. Under the applied reaction conditions, lower aging temperatures generally afforded PMO[KIT-5] materials with increased BET surface area (max. 840 m² g⁻¹) and pore volume (max. 0.93 cm³ g⁻¹), however, similar pore size (ca. 3.0 nm). Lower divalent surfactant concentrations (C_16-3-1) caused a mesophase transformation from cubic Fm3m to hexagonal P6mm symmetry. Upon relative increase of the base (NaOH) concentration, the PMO mesostructure transformed from hexagonal to cubic, then from cubic to hexagonal symmetry. Surfactants [CH₃(CH₂)₁₇N(CH₃)₂(CH₂)₃N(CH₃)₃]²⁺2Br⁻ (C_18-3-1), [CH₃(CH₂)₁₅N(CH₃)₂(CH₂)₃-N(C₂H₅)₃]²⁺2Br⁻ (C_16-3-2), and [CH₃(CH₂)₁₅N(CH₃)₃]⁺Br⁻ (C₁₆) were found to encode for PMO[SBA-1]-n and PMO[MCM-41]-n with cubic Pm3n (former) and hexagonal P6mm symmetry (latter two), respectively. All these periodic mesoporous organosilicas were characterized by powder X-ray diffraction (PXRD), nitrogen physisorption, FTIR spectroscopy, ¹³C and ²⁹Si solid state NMR spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).